Substrate-guided optical devices

ABSTRACT

There is provided an optical device, having a light-transmitting substrate having at least two major surfaces ( 26 ) parallel to each other and edges; optical means ( 16 ) for coupling light waves located in a field-of-view into the substrate by internal reflection, and at least one partially reflecting surface ( 22 ) located in the substrate which is non-parallel to the major surfaces of the substrate, characterized in that at least one of the major surfaces is coated with a dichroic coating.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 10/561,505,filed Dec. 19, 2005 for SUBSTRATE-GUIDED OPTICAL DEVICES, which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to substrate-guided optical devices, andparticularly to devices which include a plurality of reflecting surfacescarried by a common light-transmissive substrate, also referred to as alight-guide.

The invention can be implemented to advantage in a large number ofimaging applications, such as, for example, head-mounted and head-updisplays (HUD's and HUD's), cellular phones, compact displays, 3-Ddisplays, compact beam expanders, as well as non-imaging applicationssuch as flat-panel indicators, compact illuminators and scanners.

BACKGROUND OF THE INVENTION

One of the important applications for compact optical elements is inHMD's wherein an optical module serves both as an imaging lens and acombiner, in which a two-dimensional display is imaged to infinity andreflected into the eye of an observer. The display can be obtaineddirectly from either a spatial light modulator (SLM) such as a cathoderay tube (CRT), a liquid crystal display (LCD), an organic lightemitting diode array (OLED), or a scanning source and similar devices,or indirectly, by means of a relay lens or an optical fiber bundle. Thedisplay comprises an array of elements (pixels) imaged to infinity by acollimating lens and transmitted into the eye of the viewer by means ofa reflecting or partially reflecting surface acting as a combiner fornon-see-through and see-through applications, respectively. Typically, aconventional, free-space optical module is used for these purposes. Asthe desired field-of-view (FOV) of the system increases, such aconventional optical module becomes larger, heavier and bulkier,’ andtherefore, even-, for moderate performance devices, impractical. This isa major drawback for all kinds of displays but especially inhead-mounted applications, wherein the system must necessarily be aslight and as compact as possible.

The strive for compactness has led to several different complex opticalsolutions, all of which, on the one hand, are still not sufficientlycompact for most practical applications, and, on the other hand, suffermajor drawbacks in terms of manufacturability. Furthermore, theeye-motion-box (EMB) of the optical viewing angles resulting from thesedesigns is usually very small—typically less than 8 mm. Hence, theperformance of the optical system is very sensitive, even to smallmovements of the optical system relative to the eye of the viewer, anddoes not allow sufficient pupil motion for conveniently reading textfrom such displays.

DISCLOSURE OF THE INVENTION

The present invention facilitates the structure and fabrication of verycompact light-guide optical elements (LOE) for, amongst otherapplications, head-mounted displays. The invention allows relativelywide FOV's together with relatively large EMB values. The resultingoptical system offers a large, high-quality image, which alsoaccommodates large movements of the eye. The optical system offered bythe present invention is particularly advantageous because it issubstantially more compact than state-of-the-art implementations and yetit can be readily incorporated, even into optical systems havingspecialized configurations.

The invention also enables the construction of improved HUD's. HUD'shave become popular and they now play an important role, not only inmost modern combat aircrafts, but also in civilian aircrafts, in whichHUD systems have become a key component for low-visibility landingoperation. Furthermore, there have recently been numerous proposals anddesigns for HUD's in automotive applications where they can potentiallyassist the driver in driving and navigation duties. Nevertheless,state-of-the-art HUD's suffer several significant drawbacks. All HUD'sof the current designs require a display source that must be offset asignificant distance from the combiner to ensure that the sourceilluminates the entire combiner surface. As a result, thecombiner-projector HUD system is necessarily bulky and large, andrequires considerable installation space, making it inconvenient forinstallation and, at times, even unsafe to use. The large opticalaperture of conventional HUDs also pose a significant optical designchallenge, rendering the HUD's with either a compromising performance,or leading to high cost wherever high-performance is required. Thechromatic dispersion of high-quality holographic HUD's is of particularconcern.

An important application of the present invention relates to itsimplementation in a compact HUD, which alleviates the aforementioneddrawbacks. In the HUD design of the current invention, the combiner isilluminated with a compact display source that can be attached to thesubstrate. Hence, the overall system is very compact and can readily beinstalled in a variety of configurations for a wide range ofapplications. In addition, the chromatic dispersion of the display isnegligible and, as such, can operate with wide spectral sources,including a conventional white-light source. In addition, the presentinvention expands the image so that the active area of the combiner canbe much larger than the area that is actually illuminated by the lightsource.

A further application of the present invention is to provide a compactdisplay with a wide FOV for mobile, hand-held application such ascellular phones. In today's wireless Internet-access market, sufficientbandwidth is available for full video transmission. The limiting factorremains the quality of the display within the end-user's device. Themobility requirement restricts the physical size of the displays, andthe result is a direct-display with a poor image viewing quality. Thepresent invention enables, a physically very compact display with a verylarge virtual image. This is a key feature in mobile communications, andespecially for mobile Internet access, solving one of the mainlimitations for its practical implementation. Thereby, the presentinvention enables the viewing of the digital content of a full formatInternet page within a small, hand-held device, such as a cellularphone.

A broad object of the present invention, therefore, is to alleviate thedrawbacks of state-of-the-art compact optical display devices and toprovide other optical components and systems having improvedperformance, according to specific requirements.

The invention therefore provides an optical device, comprising alight-transmitting substrate having at least two major surfaces parallelto each other and edges; optical means for coupling light waves locatedin a field-of-view into said substrate by internal reflection, and atleast one partially reflecting surface located in said substrate whichis non-parallel to said major surfaces of the substrate, characterizedin that at least one of said major surfaces is coated with a dichroiccoating.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with certain preferredembodiments, with reference to the following illustrative figures sothat it may be more fully understood.

With specific reference to the figures in detail, it is stressed thatthe particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention. The description taken with the drawings are to serve asdirection to those skilled in the art as to how the several forms of theinvention may be embodied in practice.

In the drawings:

FIG. 1 is a side view of a prior art folding optical device;

FIG. 2 is a side view of an embodiment of a LOE, in accordance with thepresent invention;

FIGS. 3A and 3B illustrate the desired reflectance and transmittancecharacteristics of selectively reflecting surfaces used in the presentinvention for two ranges of incident angles;

FIG. 4 illustrates the reflectance curves as a function of wavelengthfor an exemplary dichroic coating for P-polarization;

FIG. 5 illustrates a reflectance curve as a function of wavelength foran exemplary dichroic coating for S-polarization;

FIG. 6 illustrates the reflectance curves as a function of incidentangle for an exemplary dichroic coating;

FIG. 7 is a diagram illustrating detailed sectional views of anexemplary array of selectively reflective surfaces;

FIG. 8 illustrates the reflectance curves as a function of incidentangle for an another dichroic coating;

FIG. 9 illustrates an exemplary embodiment of the present inventionembedded in a standard eye-glasses frame, and

FIG. 10 illustrates an exemplary HUD system in accordance with thepresent invention;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a conventional folding optics arrangement, whereinthe substrate 2 is illuminated by a display source 4. The display iscollimated by a collimating lens 6. The light from the display source 4is coupled into substrate 2 by a first reflecting surface 8, in such away that the main ray 10 is parallel to the substrate plane. A secondreflecting surface 12 couples the light out of the substrate and intothe eye of a viewer 14. Despite the compactness of this configuration,it suffers significant drawbacks; in particular only a very limited FOVcan be affected. As shown in FIG. 1, the maximum allowed off-axis angleinside the substrate is:

$\begin{matrix}{{\alpha_{\max} = {{arc}\; {\tan\left( \frac{T - d_{eye}}{2\; l} \right)}}},} & (1)\end{matrix}$

wherein T is the substrate thickness;

d_(eye) is the desired exit-pupil diameter, and

l is the distance between reflecting surfaces 8 and 12.

With angles higher than α_(max) the rays are reflected from thesubstrate surface before arriving at the reflecting surface 12. Hence,the reflecting surface 12 will be illuminated at an undesired directionand ghost images appear.

Therefore, the maximum achievable FOV with this configuration is:

FOV_(max)≈2vα_(max),  (2)

wherein v is the refractive index of the substrate.Typically the refractive index values lie in the range of 1.5-1.6.

Commonly, the diameter of the eye pupil is 2-6 mm. To accommodatemovement or misalignment of the display, a larger exit-pupil diameter isnecessary. Taking the minimum desirable value at approximately 8 to 10mm, the distance between the optical axis of the eye and the side of thehead, l, is, typically, between 40 and 80 mm. Consequently, even for asmall FOV of 8°, the desired substrate thickness would be of the orderof 12 mm.

Methods have been proposed to overcome the above problem. These include,utilizing a magnifying telescope inside the substrate and non-parallelcoupling directions. Even with these solutions, however, and even ifonly one reflecting surface is considered, the system thickness remainslimited by a similar value. The FOV is limited by the diameter of theprojection of the reflective surface 12 on the substrate plane.Mathematically, the maximum achievable FOV, due to this limitation, isexpressed as:

$\begin{matrix}{{{FOV}_{\max} \approx \frac{{T\; \tan \; \alpha_{sur}} - d_{eye}}{R_{eye}}},} & (3)\end{matrix}$

wherein α_(sur) is the angle between the reflecting surface and thenormal to the substrate plane, and

R_(eye) is the distance between the eye of the viewer and the substrate(typically, about 30-40 mm).

Practically tan α_(sur) cannot be much larger than 1; hence, for thesame parameters described above for a FOV of 8°, the required substratethickness here is on the order of 7 mm, which is an improvement on theprevious limit. Nevertheless, as the desired FOV is increased, thesubstrate thickness increases rapidly. For instance, for desired FOVs of15° and 30° the substrate limiting thickness is 18 mm or 25 mm,respectively.

To alleviate the above limitations, the present invention utilizes anarray of selectively reflecting surfaces, fabricated within a LOE. FIG.2 illustrates a sectional view of an LOE according to the presentinvention. The first reflecting surface 16 is illuminated by acollimated input plane wave 18, emanating from a display light source(not shown) located behind the device, wherein the plane wave 18 is oneof a set of light waves located within a given FOV to be coupled intothe LOE. The reflecting surface 16 reflects the incident light from thesource such that the light is trapped inside a planar substrate 20 bytotal internal reflection. After several reflections off the surfaces ofthe substrate, the trapped wave reaches an array of selectivelyreflecting surfaces 22, which couple the light wave 23 out of thesubstrate into the EMB 24 of a viewer. For avoiding ghost images, theoutput light wave 23 should be a plane wave, otherwise, different raysrepresenting a single point at the display source will arrive at the EMB24 of the viewer at different incident angles and ghost images thatinterfere with the primary image will be seen by the viewer. In order toprevent this phenomenon, the output light wave 23, and hence the inputwave 18, should be plane waves. That is, the angular deviation betweentwo different rays located on the same light wave should be less thanα_(res), wherein α_(res) is the angular resolution of the opticaldevice. Usually, for most visual systems, α_(res) is ˜1-2 milliradians,but different devices can yield different angular resolutions.

Assuming that the central wave of the source is coupled out of thesubstrate 20 in a direction normal to the substrate surface 26, and theoff-axis angle of the coupled wave inside the substrate 20 is α_(in),then the angle α_(sur2) between the reflecting surfaces and thesubstrate plane is:

$\begin{matrix}{\alpha_{{sur}\; 2} = {\frac{\alpha_{i\; n}}{2}.}} & (4)\end{matrix}$

As can be seen in FIG. 2, the trapped rays arrive at the reflectingsurfaces from two distinct directions 28, 30. In this particularembodiment, the trapped rays arrive at the reflecting surface from oneof these directions 28 after an even number of reflections from thesubstrate surfaces 26, wherein the incident angle β_(ref) between thetrapped ray and the normal to the reflecting surface is:

$\begin{matrix}{\beta_{ref} = {{\alpha_{i\; n} - \alpha_{{sur}\; 2}} = \frac{\alpha_{i\; n}}{2}}} & (5)\end{matrix}$

The trapped rays arrive at the reflecting surface from the seconddirection 30 after an odd number of reflections from the substratesurfaces 26, where the off-axis angle is α′_(in)=180°−α_(in) and theincident angle between the trapped ray and the normal to the reflectingsurface is:

$\begin{matrix}{\beta_{ref}^{\prime} = {{\alpha_{i\; n}^{\prime} - \alpha_{{sur}\; 2}} = {{{180{^\circ}} - \alpha_{i\; n} - \alpha_{{sur}\; 2}} = {{180{^\circ}} - {\frac{3\; \alpha_{i\; n}}{2}.}}}}} & (6)\end{matrix}$

In order to prevent undesired reflections and ghost images, thereflectance for one of these two directions should be negligible. Thedesired discrimination between the two incident directions can beachieved if one angle is significantly smaller then the other one. It ispossible to provide a coating with very low reflectance at high incidentangles and a high reflectance for low incident angles. This property canbe exploited to prevent undesired reflections and ghost images byeliminating the reflectance in one of the two directions. For examplechoosing β_(ref)˜25° from Equations (5) and (6) it can be calculatedthat:

β′_(ref)=105°;α′_(in)=50°;α′_(in)=130°;α_(sur2)=25°.  (7)

If now a reflecting surface is determined for which β′_(ref) is notreflected but β_(ref) is, the desired condition is achieved. FIGS. 3Aand 3B illustrate the desired reflectance behavior of selectivelyreflecting surfaces. While the ray 32 (FIG. 3A), having an off-axisangle of β_(ref)˜25°, is partially reflected and is coupled out of thesubstrate 34, the ray 36 (FIG. 3B), which arrives at an off-axis angleof β′_(ref)˜75° to the reflecting surface (which is equivalent toβ′_(ref)˜105°), is transmitted through the reflecting surface 34 withoutany notable reflection.

FIGS. 4 and 5 show the reflectance curves of a dichroic coating designedto achieve the above reflectance characteristics, for four differentincident angles: 20°, 25°, 30° and 75°, with P-polarized and S-polarizedlight respectively. While the reflectance of the high-angle ray isnegligible over the entire relevant spectrum, the rays at off-axisangles of 20°, 25° and 30°, obtain almost constant reflectance of 26%,29% and 32% respectively, for P-polarized light, and 32%, 28% and 25%respectively, for S-polarized light, over the same spectrum. Evidently,reflectance decreases with the obliquity of the incident rays forP-polarized light and increases for S-polarized light.

FIG. 6 illustrates the reflectance curves of the same dichroic coating,as a function of the incident angle for both polarizations at wavelengthλ=550 nm. There are two significant regions in this graph: between 65°and 80° where the reflectance is very low, and between 15° and 40° wherethe reflectance changes monotonically with decreasing incident angles(increasing for P-polarized light and decreasing for S-polarized light).Hence, as long as one can ensure that the entire angular spectrum ofβ′_(ref), where very low reflections are desired, will be located insidethe first region, while the entire angular spectrum of β_(ref), wherehigher reflections are required, will be located inside the secondregion, for a given FOV, one can ensure the reflection of only onesubstrate mode into the eye of the viewer and a ghost-free image.

There are some differences between the behaviors of the twopolarizations. The main differences are that the region of high angles,where the reflectance is very low, is much narrower for theS-polarization and that it is much more difficult to achieve a constantreflectance for a given angle over the entire spectral bandwidth for theS-polarized light than for the P-polarized light. It is thereforepreferable to design the LOE only for the P-polarized light. This wouldbe satisfactory for a system using a polarized display source, such asan LCD, or for a system where the output brightness is not crucial andthe S-polarized light can be filtered out. However, for an unpolarizeddisplay source, like a CRT or an OLED, or for a system where thebrightness is critical, S-polarized light cannot be neglected and itmust be taken into account during the design procedure. Anotherdifference is that the monotonic behavior of the S-polarized light atthe angular spectrum of β_(ref), where higher reflections are required,is opposite to that of the P-polarized light, that is, the reflectancefor the S-polarized light increases with the obliquity of the incidentrays. This contradictory behavior of the two polarizations at theangular spectrum of β_(ref) could be utilized during the optical designof the system to achieve the desired reflectance of the overall lightaccording to the specific requirements of each system.

Assuming that the coupled wave illuminates the entire area of thereflecting surface, after reflection from the surface 16, it illuminatesan area of 2S₁=2T tan(α) on the substrate surface. On the other hand,the projection of a reflection surface 22 on the substrate plane, isS₂=T tan(α_(sur2)). To avoid either overlapping or gaps between thereflecting surfaces, the projection of each surface is adjacent to itsneighbor. Hence, the number N of reflecting surfaces 22 through whicheach coupled ray passes during one cycle (i.e., between two reflectionsfrom the same surface of the substrate) is:

$\begin{matrix}{N = {\frac{2\; S_{1}}{S_{2}} = {\frac{2\; {T \cdot {\cot \left( \alpha_{{sur}\; 1} \right)}}}{T \cdot {\cot \left( \alpha_{{sur}\; 2} \right)}}.}}} & (8)\end{matrix}$

In this example, where α_(sur2)=25° and α_(sur1)=25°, the solution isN=2; that is, each ray passes through two different surfaces during onecycle.

The embodiment described above with regard to FIG. 7 is an example of amethod for coupling the input waves into the substrate. Input wavescould, however, also be coupled into the substrate by other opticalmeans, including, but not limited to, folding prisms, fiber opticbundles, diffraction gratings, and other solutions.

Also, in the example illustrated in FIG. 2, the input waves and theimage waves are located on the same side of the substrate. Otherconfigurations are envisioned in which the input and the image wavescould be located on opposite sides of the substrate. It is alsopossible, in certain applications, to couple the input waves into thesubstrate through one of the substrate's peripheral sides.

FIG. 7 is a detailed sectional view of an array of selectivelyreflective surfaces which couple light, trapped inside the substrate,out and into the eye of a viewer. As can be seen, in each cycle thecoupled ray passes through reflecting surfaces 43, at an angle ofα′_(in)=130°, whereby the angle between the ray and the normal to thereflecting surfaces is ˜75°. The reflections from these surfaces arenegligible. In addition, the ray passes twice through the reflectingsurface 44, in each cycle, at an angle of α_(in)=50°, where the incidentangle is 25°. Part of the energy of the ray is coupled out of thesubstrate. Assuming that one array of two selectively reflectingsurfaces 22 is used to couple the light onto the eye of a viewer, themaximal FOV is:

$\begin{matrix}{{FOV}_{\max} \approx {\frac{{2\; T\; \tan \; \alpha_{{sur}\; 1}} - d_{eye}}{R_{eye}}.}} & (9)\end{matrix}$

Hence, for the same parameters of the examples above, the limitingsubstrate thickness for an FOV of 8° is in the order of 2.8 mm; for FOVsof 15° and 30°, the limiting substrate thickness is 3.7 mm and 5.6 mm,respectively. These are more favorable values than the limitingthickness of the state-of-the-art solutions discussed above. Moreover,more than two selectively reflecting surfaces can be used. For example,for three selectively reflecting surfaces 22, the limiting substratethickness for FOVs of 15° and 30° is approximately 2.4 mm and 3.9 mm,respectively. Similarly additional reflecting surfaces may be introducesto, amongst other advantages, reduce the limiting optical thicknessfurther.

For configuration where a relatively small FOV is required, a singlepartially reflecting surface can be sufficient. For example, for asystem with the following parameters: R_(eye)=25 mm; α^(sur)=72° and T=5mm, a moderate FOV of 17° can be achieved even with a single reflectingsurface 22. Part of the rays will cross the surface 22 several timesbefore being coupled out into the desired direction. Since the minimalpropagation angle inside the substrate to achieve the total-internalreflection condition for BK7 material or similar is α_(in(min))=42°, thepropagation direction of the central angle of the FOV isα_(in(cen))=48°. Consequently, the projected image is not normal to thesurface but is rather inclined to 12° off-axis. Nevertheless, for manyapplications this is acceptable.

Unfortunately, this solution is not always feasible. For many otherapplications there is a constraint that the projected image should benormal to the substrate surface. Another problem, which is associatedwith the total internal reflection condition, is the maximal FOV of theimage that can be trapped inside the substrate. Unfortunately, it isvery difficult to achieve very low reflectance for off-axis anglesexceeding 82°. Assuming that the required FOV angle inside the substrateis α_(FOV), the maximal incident angle between the central wave and thenormal to the reflecting surface is

$\begin{matrix}{\beta_{ref}^{\prime} = {{82{^\circ}} - {\frac{\alpha_{FOV}}{2}.}}} & (10)\end{matrix}$

Assuming an external FOV of 30°, which corresponds to α_(FOV)˜20° insidethe substrate, yields β′ref=72°. Inserting this value into Eq. (6)yields α_(in)=48°, and hence the minimal required angle of the trappedwave is

$\begin{matrix}{\alpha_{i\; {n{(\min)}}} = {{\alpha_{i\; n} - \frac{\alpha_{FOV}}{2}} = {38{{^\circ}.}}}} & (11)\end{matrix}$

Clearly, this angle cannot be trapped inside BK7 or other similarmaterials. It is true that there are flint optical materials with higherrefractive indices, which can exceed 1.8, however, the transparency ofthese materials is usually not high enough for substrate-mode opticalelements. Another possible solution is to coat the substrate surfacesnot with regular anti-reflection coatings but with angular-sensitivereflecting coatings that trap the entire FOV inside the substrate evenfor lower angles than the critical angle. It must be noted that even fora non see-through applications, where one of the substrate surfaces canbe opaque and hence can be coated with a conventional reflectingsurface, the other surface, the one which is next to the eyes of theviewer, should be transparent, at least for the angles of the requiredexternal FOV. Therefore, the required reflecting coating should havevery high reflectance for the region of angles lower than the criticalangle and very high reflectance for the entire FOV of the image.

FIG. 8 shows the reflectance curves of a dichroic coating designed toachieve the above reflectance characteristics, as a function of theincident angle, for both polarizations at the wavelength λ=550 nm, wherethe angle is measured in air. Evidently, there are two significantregions in this graph: between 30° and 90° (equivalent to 20°-42° insidethe substrate) where the reflectance is very high; and between 0° and22° (equivalent to 0°-15° inside the substrate) where the reflectance isvery low. Hence, as long as one can ensure that the entire angularspectrum of α_(in), where very high reflections are desired, will belocated inside the first region, while the entire angular spectrum ofexterior FOV, where essentially zero reflections are required, will belocated inside the second region, for a given FOV, one can ensure thatthe entire FOV will be trapped inside the substrate by internalreflections and that the viewer can see the whole image. It is importantto note that since the fabrication process of the LOE usually involvescementing optical elements and since the required angular-sensitivereflecting coating is applied to the substrate surface only after theLOE body is complete, it is not possible to utilize the conventionalhot-coating procedures that may damage the cemented areas. Fortunately,novel thin-film technologies, as ion-assisted coating procedures, canalso be used for cold processing. Eliminating the need to heat partsallows cemented parts, such as LOEs, to be safely coated.

In general, LOE offer several important advantages over alternativecompact optics for display applications, which include:

1) The input display source can be located very close to the substrate,so that the overall optical system is very compact and lightweight,offering an unparalleled form-factor.2) In contrast to other compact display configurations, the presentinvention offers flexibility as to location of the input display sourcerelative to the eyepiece. This flexibility, combined with the ability tolocate the source close to the expanding substrate, alleviates the needto use an off-axis optical configuration that is common to other displaysystems. In addition, since the input aperture of the LOE is muchsmaller than the active area of the output aperture, the numericalaperture of the collimating lens 6 is much smaller than required for acomparable conventional imaging system. Consequently a significantlymore convenient optical system can be implemented and the manydifficulties associated with off-axis optics and high numerical-aperturelenses, such as field or chromatic aberrations can be compensated forrelatively easily and efficiently.3) The reflectance coefficients of the selectively reflective surfacesin the present invention are essentially identical over the entirerelevant spectrum. Hence, both monochromatic and polychromatic, lightsources may be used as display sources. The LOE has a negligiblewavelength-dependence ensuring high-quality color displays with highresolutions.4) Since each point from the input display is transformed into a planewave that is reflected into the eye of the viewer from a large part ofthe reflecting array, the tolerances on the exact location of the eyecan be significantly relaxed. As such, the viewer can see the entireFOV, and the EMB can be significantly larger than in other compactdisplay configurations.5) Since a large part of the intensity from the display source iscoupled into the substrate, and since a large portion of this coupledenergy is “recycled” and coupled out into the eye of the viewer, adisplay of comparatively high brightness can be achieved even withdisplay sources with low power consumption.

FIG. 9 illustrates an embodiment of the present invention in which theLOE 20 is embedded in an eye-glasses frame 58. The display source 4, thecollimating lens 6, and the folding lens 60 are assembled inside the armportions 62 of the eye-glasses frame, just next to the edge of the LOE20. For a case in which the display source is an electronic element suchas a small CRT, LCD, or OLED, the driving electronics 64 for the displaysource might be assembled inside the back portion of the arm 62. A powersupply and data interface 66 is connectable to arm 62 by a lead 68 orother communication means including radio or optical transmission.Alternatively, a battery and miniature data link electronics can beintegrated in the eye-glasses frame.

The embodiment described above can serve in both see-through andnon-see-through systems. In the latter case opaque layers are located infront of the LOE. It is not necessary to occlude the entire LOE,typically only the active area, where the display is visible needs to beblocked. As such, the device can ensure that the peripheral vision ofthe user is maintained, replicating the viewing experience of a computeror a television screen, in which such peripheral vision serves animportant cognitive function. Alternatively, a variable filter can beplaced in front of the system in such a way that the viewer can controlthe level of brightness of the light emerging from the external scene.This variable filter could be either a mechanically controlled devicesuch as a folding filter, or two rotating polarizers, an electronicallycontrolled device, or even an automatic device, whereby thetransmittance of the filter is determined by the brightness of theexternal background. One method to achieve the required variabletransmittance filter is to use electrochromic materials in order toprovide electrical control of optical transmittance, wherein materialswith electrically controllable optical properties are incorporated intolaminated structures.

There are some alternatives as to the precise way in which an LOE can beutilized in this embodiment. The simplest option is to use a singleelement for one eye. Another option is to use an element and a displaysource for each eye, but with the same image. Alternatively it ispossible to project two different parts of the same image, with someoverlap between the two eyes, enabling a wider FOV. Yet anotherpossibility is to project two different scenes, one to each eye, inorder to create a stereoscopic image. With this alternative, attractiveimplementations are possible, including 3-dimensional movies, advancedvirtual reality, training systems and others.

The embodiment of FIG. 9 is just an example illustrating the simpleimplementation of the present invention. Since the substrate guidedoptical element, constituting the core of the system, is very compactand lightweight, it could be installed in a vast variety ofarrangements. Hence many other embodiments are also possible including avisor, a folding display, a monocle, and many more. This embodiment isdesignated for applications where the display should be near-to-eye:head-mounted, head-worn or head-carried.

The embodiment described above is a mono-ocular optical system, that is,the image is projected onto a single eye. There are, however,applications, such as head-up displays (HUD), wherein it is desired toproject an image onto both eyes. Until recently, HUD systems have beenused mainly in advanced combat and civilian aircraft. There have beennumerous proposals and designs, of late, to install a HUD in front of acar driver in order to assist in driving navigation or to project athermal image into his eyes during-low-visibility conditions. Currentaerospace HUD systems are very expensive, the price of a single unitbeing in the order of hundreds of thousands of dollars. In addition, theexisting systems are very large, heavy, and bulky, and are toocumbersome for installation in a small aircraft let alone a car.LOE-based HUD potentially provide the possibilities for a very compact,self-contained HUD, that can be readily installed in confined spaces. Italso simplifies the construction and manufacturing of the opticalsystems related to the HUD and therefore is a potentially suitable forboth improving on aerospace HUD's, as well as introducing a compact,inexpensive, consumer version for the automotive industry.

FIG. 10 illustrates a method of materializing an HUD system based on thepresent invention. The light from a display source 4 is collimated by alens 6 to infinity and coupled by the first reflecting surface 16 intosubstrate 20. After reflection at a second reflecting array (not shown),the optical waves impinge on a third reflecting surfaces 22, whichcouples the light out into the eyes 24 of the viewer. The overall systemcan be very compact and lightweight, of the size of a large postcardhaving a thickness of a few millimeters. The display source, having avolume of a few cubic centimeters, can be attached to one of the cornersof the substrate, where an electric wire can transmit the power and datato the system. It is expected that the installation of the presented HUDsystem will not be more complicated than the installation of a simplecommercial audio system. Moreover, since there is no need for anexternal display source for image projection, the necessity to installcomponents in unsafe places is avoided.

The embodiments illustrated in FIG. 10 can be implemented for otherapplications, in addition to HUD systems for vehicles. One possibleutilization of these embodiments is as a flat display for a computer ortelevision. The main unique characteristic of such a display is that theimage is not located at the screen plane, but is focused at infinity orto a similarly convenient distance. One of the main drawbacks ofexisting computer displays is that the user has to focus his eyes at avery close distance of between 40 and 60 cm, while the natural focus ofa healthy eye is to infinity. Many people suffer from headaches afterworking for a long duration of time at a computer. Many others who workfrequently with computers tend to develop myopia. In addition, somepeople, who suffer from both myopia and hyperopia, need specialspectacles for work with a computer. A flat display, based on thepresent invention, could be an appropriate solution for people whosuffer from the above-described problems and do not wish to work with ahead-mounted display. Furthermore, the present invention allows for asignificant reduction in the physical size of the screen. As the imageformed by the LOE is larger than the device, it would be possible toimplement large screens on smaller frames. This is particularlyimportant for mobile applications such as lap and palm-top computers.

Yet another possible implementation of this embodiment is as a screenfor a personal digital assistance (PDA). The size of the existingconventional screens which are presently used, is under 10 cm. Since theminimal distance where these displays can be read is on the order of 40cm, the obtainable FOV is under 15°; hence, the information content,especially as far as text is concerned, on these displays is limited. Asignificant improvement in the projected FOV can be made with theembodiment illustrated in FIG. 10. The image is focused at infinity, andthe screen can be located much closer to the eyes of the viewer. Inaddition, since each eye sees a different part of the totalfiled-of-view (TFOV), with an overlap at its center, another increase inthe TFOV may be achieved. Therefore, a display with an FOV of 40° orlarger is feasible.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

1. An optical device, comprising: a light-transmitting substrate havingat least two major surfaces parallel to each other and edges; a firstoptical element for coupling light waves located in a field-of-view intosaid substrate by internal reflection; and a second optical element forcoupling said trapped light waves out of said substrate, characterizedin that at least one of said major surfaces is coated with an angularsensitive coating.
 2. The optical device according to claim 1, whereinsaid first optical element is a wave-reflecting surface.
 3. The opticaldevice according to claim 1, wherein said first optical element is afolding prism.
 4. The optical device according to claim 1, wherein saidfirst optical element is a diffraction grating.
 5. The optical deviceaccording to claim 1, wherein said second optical element is located insaid substrate.
 6. The optical device according to claim 1, wherein saidat least one coated major surface has a negligible reflection for onepart of the angular spectrum and a noticeable reflection for other partsof the angular spectrum.
 7. The optical device according to claim 1,wherein said at least one coated major surface has a low reflectance atlow incident angles and a high reflectance at high incident angles. 8.The optical device according to claim 1, wherein said angular sensitivecoating causes said field-of-view to be trapped inside the substrate byinternal reflections.
 9. The optical device according to claim 1,wherein said two major surfaces are coated with an angular sensitivecoating.
 10. The optical device according to claim 9, wherein said twomajor surfaces are coated with the same angular sensitive coating. 11.The optical device according to claim 1, wherein said second opticalelement causes said field-of-view to exit said substrate at apredetermined location for reaching at least one eye of an observer. 12.The optical device according to claim 1, wherein said angular sensitivecoating is formed by utilizing an ion-assisted coating procedure. 13.The optical device according to claim 1, further comprising a displaylight source.
 14. The optical device according to claim 1, wherein saidsubstrate is partially transparent, to enable see-through operation. 15.The optical device according to claim 1, wherein said device is mountedin an eyeglasses frame.
 16. The optical device according to claim 1,wherein said device is located in a head-up-display.